Kinesthetic motor imagery and spinal excitability: The effect of contraction intensity and spatial localization

Objective Data on whether motor imagery (MI) modulates spinal excitability are equivocal. The purpose of this study was to determine if imagined muscle contractions of the left plantar flexor (PF) alter spinal excitability, and if so, to determine whether this alteration is intensity dependent and/or localized to the target muscles. Our research questions required two experiments. Methods In experiment 1, 16 healthy volunteers performed imagined muscle contractions using a kinesthetic approach with their left PF at 25% and 100% of imagined effort (IE). The soleus H-reflex was evoked during three conditions, which were separated by about 15 s: rest (preceding MI), during MI, and recovery (following the cessation of MI). In experiment 2, a subset of subjects from experiment 1 performed MI with their left PF at 100% of IE, while either the soleus or flexor carpi radialis (FCR) H-reflex was measured. Results In experiment 1, we observed a facilitation of soleus H-wave amplitude during MI compared to the rest and recovery conditions (p < 0.05). Furthermore, the soleus H-wave amplitude was greater during 100% than 25% of IE (p < 0.05). In experiment 2, soleus and FCR H-wave amplitude increased during imagined muscle contractions of the left PF (p < 0.05). These changes were independent of voluntary muscle activity. Conclusions These findings suggest MI can increase spinal excitability by the intensity of imagined effort, but this effect is not fully localized to the task specific muscle. Significance These data provide evidence that MI can increase spinal excitability in healthy subjects, which suggests future studies are warranted to examine the clinical relevance of this effect. These studies are needed to help establish a therapeutic theory by which to advance motor function rehabilitation using MI.